1. Field of the Invention
The present invention relates to a method of manufacturing semiconductor devices and, more particularly, to a method of manufacturing semiconductor devices having trench isolation.
2. Description of Related Art
For highly integrated semiconductor devices, an isolation area between circuit elements, such as transistors, needs to be as narrow as possible to enhance integration. The conventional isolation based on LOCOS (Local Oxidation of Silicon) cannot prevent a bird's beak defect from occurring, thereby failing to narrow the isolation area any further. Highly integrated devices also require multi-level interconnection technology. Implementation of this technology primarily requires planarization of the semiconductor substrate to prevent interconnected wiring from being disconnected at steps produced by raggedness for example, if any, on the substrate. Thus, planarization is often required in manufacturing semiconductor devices. To obtain the best planarization result, planarization needs to be practiced starting with as early a manufacturing step as possible. To meet this requirement, planar trench isolation that enables inter-element isolation with a relatively small area is produced. In trench isolation, trenches formed on the semiconductor substrate are filled up with an isolation material to provide inter-element isolation. This method is effective in forming trenches microscopically; excess filling material forming protrusions, however needs to be removed flatly.
FIGS. 1A through 1C show a method of forming trench isolation. In this method, a trench 3 is first formed outside the element forming area on the substrate as shown in FIG. 1A and then the formed trench is filled up with a filling material 4 by a depositing process such as CVD (Chemical Vapor Deposition) as shown in FIG. 1B. In the process, the filling material is deposited thick also outside the trench 3 to form a protrusion 8. This protrusion 8 is abraded planar as shown in FIG. 1C. Reference numeral 2 indicates a stopper layer serving to block abrasion. If the filling material is SiO.sub.2 for example, the stopper layer 2 is formed by a material, Si.sub.3 N.sub.4 for example, which is lower than SiO.sub.2 in the rate at which the material is abraded.
If, in the above-mentioned method, a wide protruding area 8a, a wide protruding area 8b caused by closely arranged narrow recesses, and a wide area 9 are formed by the trench filling as shown in FIG. 2A, then performing abrasion on the protruding areas with the trench 3 filled causes abrasion pattern dependency as shown in FIG. 2B. In the wide recess area 9, film thinning is caused by the deformation of abrasive cloth during abrasion. In the wide protruding area 8a and the area 8b where the wide protrusion has been formed after filling, a filling material 4a is left unabraded at generally the center of each of these areas. Therefore, in the following process, when the stopper layer 2 made of Si.sub.3 N.sub.4 for example is removed by hot phosphoric acid for example, the filling material 4a made of SiO.sub.2 and the like on Si.sub.3 N.sub.4 is detached, causing particles. If too much abrasion is performed to attempt the total removal of the excess filling material, the film in the wide recess area is thinned even further, losing planarity.
To overcome this problem, J. Vac. Technol. B12 (1), January/February 1994, pp. 54 through 57 proposes a technique as shown in FIGS. 3A through 3H. As shown, an SiO.sub.2 film and a stopper layer 2 made of Si.sub.3 N.sub.4 for example are formed on a substrate 1 to be patterned by lithography and dry etching. Further, anisotropic etching is performed on the isolation area where the surface of the substrate with the film removed is exposed, forming a trench 3 about 0.4 micron deep (FIG. 3A). The filling material 4 made of SiO.sub.2 is formed such that the same fills up the trench 3 (FIG. 3B), on which polycrystalline silicon 10 and then a second SiO.sub.2 film are formed (FIG. 3C). On the second SiO.sub.2 film, a photo resist 5 is formed (FIG. 3D) and patterning is performed such that the second SiO.sub.2 film remains only on the wider recess portion (FIG. 3E). The protruding polycrystalline silicon 10 is removed by abrasion with the second SiO.sub.2 film 11 serving as the stopper layer (FIG. 3F). Then, the remaining polycrystalline silicon 10 used as the mask, the filling material 4 made of SiO.sub.2 is etched (FIG. 3G). At this moment, the stopper layer 2 made of Si.sub.3 N.sub.4 works as the stopper against the etching of the filling material SiO.sub.2. Then, the protruding portions composed of the remaining polycrystalline silicon 10 and the filling material 4 are removed by abrasion for planarization (FIG. 3H). At this moment, the stopper layer 2 made of Si.sub.3 N.sub.4 works as the stopper layer against the abrasion of the filling material 4.
However, this conventional method requires to prepare an entirely new mask for patterning the second SiO.sub.2 film 11 that works as the stopper layer against the first abrasion of the polycrystalline silicon 10. This conventional method also requires relatively many manufacturing steps because film forming of the polycrystalline silicon 10 must be performed once, lithographing once, and abrasion of the polycrystalline silicon 10 twice.